Exhaust emission control devices, such as catalytic converters, achieve higher emission reduction after reaching a predetermined operating temperature. Thus, to lower vehicle emissions, various methods are possible to raise emission control device temperature as fast as possible.
In one approach to raise the emission control device temperature, an engine is operated with a rich air-fuel ratio and secondary air is directly introduced into the exhaust upstream of an emission control device. Such a system is disclosed in U.S. Pat. No. 5,136,842. In this system, the excess fuel from the engine is expelled into the exhaust, where it reacts with the secondary air upstream of the emission control device. The reacted fuel increases the temperature of the exhaust, thus heating the emission control device. However, in order to introduce the secondary air into the exhaust, a secondary air pump is used to increase the pressure of the secondary air above the exhaust pressure. Use of such a pump may degrade fuel efficiency and requires additional packaging space, thus reducing the efficiency of the engine.
The inventors herein have recognized the issues with the above approach and offer a method to at least partly address them. In one embodiment, a method for operation of a vehicle system is provided. The method includes, during a first operating condition, increasing back pressure in a first exhaust conduit positioned upstream of a turbine and downstream of a first emission control device and during a second operating condition, reducing back pressure in the first exhaust conduit and flowing boosted air from downstream of a compressor into a second exhaust conduit positioned upstream of a second emission control device and downstream of the turbine.
In this way, back pressure is generated in the exhaust system which causes an increase in the temperature of the first emission control device, increasing exhaust gas energy retention in the exhaust system and reducing emissions. Further emission reductions are achieved by flowing the boosted air, which is oxygen-rich, to the second emission control device, after the back pressure is reduced. The excess oxygen in the secondary air will react with reductants, such as unburnt fuel, in the exhaust to create an exothermic reaction that will heat the second emission control device, thereby reducing emissions. In this way, boosted air downstream of the compressor may be directed to the exhaust upstream of an emission control device without utilizing a separate pump. By utilizing the pressure differential between the throttle inlet pressure downstream of the compressor and the turbine outlet pressure near where the secondary air is introduced into the exhaust conduit, the boosted air may be driven to the exhaust without a separate pressurization mechanism. The aforementioned emission reduction techniques may be implemented during different operating conditions to effectively reduce emissions.
The present disclosure may offer several advantages. For example, by relying on boosted air rather than air pressurized by a pump, a separate air pump may be dispensed with, thus improving engine efficiency. Additionally, by increasing back pressure upstream of the turbine the first emission control device may be rapidly heated, during a cold start for example, to further reduce emissions prior to the flowing of the boosted air to the emission control device.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.